This invention relates to a method for controlling a speed reduction ratio in a continuously variable speed transmission for a vehicle.
Therefor the following methods are known for speed control of a continuously variable speed transmission: selecting as a reference value either (a) the engine speed, (b) the rate of change of engine speed, or (c) the speed reduction ratio, and controlling the speed reduction ratio so as to follow the reference value.
These control methods, however, do not take the acceleration caused by the reserved power of the engine into account. As a result, the speed reduction ratio according to the known control methods tends to be larger or smaller than expected, thus at a low vehicle speed causing: (a) time-lag and unfavorable feeling produced by the time-lag (poor response) because of a small change rate of the speed reduction ratio during ratio control toward greater speed reduction ratio, (b) poor fuel consumption or unfavorable feelings accompanying an excessive increase of engine speed during ratio control toward smaller speed reduction ratio, (c) hunting of the engine speed because of a small change rate of the speed reduction ratio during ratio control toward greater speed reduction ratio, and (d) poor fuel consumption associated with poor efficiency due to excessive reduction of the speed reduction ratio during deceleration.
Therefore, the applicant proposed a method for solving these problems which comprises; calculating a change rate of speed reduction ratio by summing up a component corresponding to a predicted acceleration and a component corresponding to a reference change rate of the engine speed, and controlling speed reduction ratio based on the calculated change rate of speed reduction ratio (as exemplified by Japanese Patent laid-open Publications No. 63(1988)-53343 and No. 63(1988)-53344).
In the above control method, however, if the change rate of speed reduction ratio is calculated based on level road running, the actual engine speed will deviate from the reference engine speed because of a difference in running resistances during running on an ascending or a descending slope. More specifically, in the ascending slope situation the actual engine speed tends to be less than the reference speed, while in the descending slope situation the actual engine speed tends to be greater than the reference speed.
Many continuously variable speed transmissions employ clutch means which can control the transmission of engine power. In the partial engaging state of the clutch means, since the whole engine power is not directly transmitted to the transmission, such speed control as described above cannot be applied. For this reason, in the prior art, the speed control is divided into one for use "during clutch engagement" and the other for use "after clutch engagement completion". In an example of the clutch control, for instance, as disclosed in Japanese Patent laid-open Publication No. 56(1981)-95722, where clutch opening is controlled based on engine throttle opening, engine speed and vehicle speed.
But, in the case where the speed reduction ratio is controlled in the two stages, "during clutch engagement" and "after clutch engagement completion", it is difficult to smoothly shift the clutch engagement control stage to the post clutch engagement completion control stage and to smoothly start the speed reduction ratio control, because it is not easy to identify the exact time of clutch engagement completion, thus deteriorating driver's feeling. And, for the speed reduction ratio control, such a control has been employed as to set a reference engine speed corresponding to an accelerator opening and to make the engine speed follow the reference engine speed. But, if the determination of clutch engagement completion is inaccurate, the start of the speed reduction ratio control based on the reference engine speed may be delayed, thus resulting in momentary abrupt increase of the engine speed or insufficient feeling of acceleration.